Prior cell switching networks typically allow integration of voice, data, facsimile, and video signals over a network, permitting the interconnection of computers, private branch exchanges ("PBXs"), and other communications equipment using a single type of switching fabric. Cell switching networks are also referred to as communication networks for fixed length packets. This prior cell technology is also known as Cell Relay by the Institute of Electrical and Electronic Engineers ("IEEE") and as Asynchronous Transfer Made ("ATM") by the International Telegraph and Telephone Consultative Committee ("CCITT").
Cell switches typically operate in private corporate networks. Private high speed digital networks have expanded relatively rapidly in the United States due to the ready availability of T1 trunks and in Europe due to the ready availability of E1 trunks. A typical international network may use a mixture of (1) T1 trunks in North America and Japan and (2) E1 trunks in Europe. Gateway nodes provide the necessary conversions between the two standards.
Unlike prior circuit switching communication networks that commit specific communication resources to a given end-to-end connection, prior cell switching networks typically establish virtual connections. This means that network resources are generally only used when needed. This allows for optimal use of network resources (bandwidth) based on the existing demands and priorities. This contrasts with dedicated physical routing that does not take into consideration varying demand and priorities.
Each cell switching node of a prior cell switching network typically permits a user to enter a message longer than the specified maximum cell size. The longer message is typically segmented and formatted into multiple cells along with "housekeeping" information presented by the network protocol. The individual cells are reassembled at the destination node. Typically there is reduced transit delay because of pipelining and reduced average queuing and packetization delay at each node because of short cell formats.
A prior North American or Japanese T1 trunk typically has a data rate of 1.544 megabits per second ("Mbps"). A prior European E1 trunk typically has a data rate of 2.048 Mbps. Prior fractional T1 trunks allow operation at integral multiples of 64 kilobits per second ("Kbps"), which is a digital signal zero "DSO" unit of bandwidth corresponding to the rate required for a single voice channel. This helps to avoid the full cost of a T1 trunk. This service is available in the U.S. and Canada. A similar European trunk service, known as X.21, supports data rates from 256 Kbps to 1.920 Mbps in units of 64 Kbps on unframed E1 lines.
The incorporation of fractional trunks into a prior cell network typically generates longer queuing delays at the nodes of these trunks. For a fixed queue length, queuing delay is inversely proportional to the trunk data rate. These delays may introduce restrictions on certain uses of these trunks. Voice communications are typically drastically hindered by excessive end-to-end delays. The management of these delays in a cell network is often critical.
Another complication resulting from the incorporation of fractional trunks into a prior cell network is that varying delays may result from dynamic route selection when there are links of differing capacities and queuing delays.